I can't answer the flat or square pack question, but I use 2/3 AAA 400 mah flat packs from cheapbatterypacks.com on all my combat birds that weigh about an ounce.

I did however order some of these for my Open B ships and they have yet to arrive. LiFe 6.6 v 700 mah and weighs about an ounce and a half. Cost less that 3 bucks.
I know of several combat flyers using this pack with good results.